1. Field of the Invention
The present invention relates to a low pressure vapor phase growth apparatus for thin film formation on the obverse surface of a substrate under reduced pressure.
2. Description of Prior Art
FIG. 2 is a schematic cross-sectional view of a conventional low pressure vapor phase growth apparatus with a direct substrate-heating system, wherein all members around a substrate 4 are in a rotary symmetrical form.
In this conventional apparatus, the substrate 4 is pressed against and fixed on the obverse surface (lower surface in the figure) of a light-transmissive supporting bed 5 by means of an annular light-transmissive fixing tool 7 provided on the upper side of an annular lifter 12 to be driven either upward or downward with operating bars 120 to be moved from the outside of a reaction chamber 1.
The substrate 4 is radiant-heated, from the reverse surface thereof, with a lamp 6 disposed on the rear side of the light-transmissive supporting bed 5, while a reactive gas is fed into the reaction chamber 1 through a gas feeder 2. The reactive gas undergoes a chemical reaction on the obverse surface (lower surface in the figure) of the substrate 4 to grow a film thereon. The unreacted gas and the resulting formed gas are pumped out through an exhauster 3.
For the purpose of keeping the light-transmissive supporting bed 5 stable at a low temperature, a light-transmissive plate 10 is set to provide a cooling chamber 8 between the plate 10 and the light-transmissive supporting bed 5. A cooling medium (water) 11 is flowed through the cooling chamber 8.
The supply of heat to the substrate 4 in this apparatus is effected only by a radiant energy directed thereto from the lamp 6 because the light-transmissive supporting bed 5 is cooled in a manner as described above.
A factor in determining the uniformities of the thickness and quality of a film being grown on the obverse surface of the substrate 4 is the uniformity of the temperature of the obverse surface of the substrate 4. In order to supply heat stably and uniformly to the substrate 4, the position and symmetry in shape of the lamp 6 as well as the shape of a rear reflective plate 60 must be appropriately designed to secure the symmetry in shape of the above-mentioned cooling chamber 8, the symmetry and uniformity of inflow and outflow of the cooling medium 11, etc.
In the foregoing conventional apparatus, however, there is a problem that the temperature of the substrate 4 is locally lowered around the light-transmissive fixing tool 7 used to fix the substrate 4 on the light-transmissive supporting bed 5 even if the foregoing conditions are all satisfied ideally to uniformize the supply of radiant heat to the substrate 4.
This problem arises totally from large differences in temperature and thermal conductivity between the substrate 4 and the light-transmissive supporting bed 5.
More specifically, since only the substrate 4 is heated and increased in temperature with a radiant energy generated by the lamp 6 and transmitted through the light-transmissive plate 10, the cooling medium (water) 11 and the light-transmissive supporting bed 5, there is an inevitable tendency for a difference in temperature to arise between the substrate 4 and the light-transmissive supporting bed 5. Specifically, the temperature of the substrate 4 is raised much more than that of the light-transmissive supporting bed 5. The difference in temperature between the substrate 4 and the light-transmissive supporting bed 5 is further enhanced because the light-transmissive supporting bed 5 is always cooled as described above. The higher the film-forming temperature on the substrate 4 is set, the larger the above-mentioned difference. Thus, the enlarged difference in temperature increases the heat flow from the reverse surface of the substrate 4 toward the light-transmissive supporting bed 5.
This heat flow is caused totally by two kinds of heat condition: heat conduction via a gas existing between the substrate 4 and the light-transmissive supporting bed 5 and heat conduction via direct contact areas therebetween.
A description will first be made of the "heat conduction via the existing gas." Gaps defined by the microscopically uneven reverse surface of the substrate 4 and the microscopically uneven obverse surface of the light-transmissive supporting bed 5 are generally at most 1 .mu.m in width. Therefore, the gas existing between the substrate 4 and the light-transmissive supporting bed 5 is in a state of molecular flow within the common range of pressure employed in vapor phase growth. Accordingly, the rate of "heat conduction via the existing gas" is proportional to both of the "difference in temperature" between the substrate 4 and the light-transmissive supporting bed 5 and the "number of molecules of the existing gas." This rate of heat conduction is believed to be uniform all over the reverse surface of the substrate 4.
A description will now be made of the "heat conduction via the contact areas" between the substrate 4 and the light-transmissive supporting bed 5. The rate of this kind of heat conduction is proportional to both of the "difference in temperature therebetween" and the "effective contact area." This "effective contact area" is increased in keeping with the increasing contact pressure between the substrate 4 and the light-transmissive supporting bed 5.
The local lowering in the temperature of the substrate 4 around the light-transmissive fixing tool 7 in the conventional apparatus is believed to be caused altogether by a corresponding local rise in the rate of heat flow through the above-mentioned "heat conduction via the contact areas" as a result of an increase in the effective contact area between the substrate 4 and the light-transmissive supporting bed 5 particularly around the light-transmissive fixing tool 7, which increase is brought about by the contact pressure applied by means of the light-transmissive fixing tool 7.
Furthermore, pressing by means of the light-transmissive fixing tool 7 also increases the effective contact area between the fixing tool 7 itself and the substrate 4 to bring about an increase in the heat flow from the substrate 4 to the fixing tool 7, which is also responsible for the local lowering in the temperature of the substrate 4 around the fixing tool 7.